1. Field of the Invention
The present invention generally relates to high precision imaging using a reflective optical element and, more particularly, to high precision lithography exposure systems and methods using one or more adaptive, reflective optical elements to minimize aberrations and measurement and control therefor.
2. Description of the Prior Art
Many manufacturing and scientific processes require the use of optical systems having extremely high accuracy and freedom from aberrations as well as the ability to make observations and/or exposures in wavelength regimes well outside the visible spectrum. For example, at least one lithographic exposure process is invariably required for establishing the location and basic dimensions of respective electrical or electronic elements in semiconductor integrated circuits in which the number of such elements on a single chip can extend into the tens if not hundreds of millions. The respective electrical or electronic elements can be very small, and placement in close proximity, sometimes referred to as high integration density, is highly desirable in order to reduce signal propagation time and susceptibility to noise as well as to achieve other advantages such as increased functionality and, in some cases, manufacturing economy. These circumstances provide strong incentives to develop smaller minimum feature size regimes which must be established through lithographic exposures of a resist. Therefore, feature resolution and fidelity of the exposure image must be held within a very closely defined budget which is a small fraction of the minimum feature size.
The resolution of any optical system is a function of the wavelength of the radiation used for the exposure, although some arrangements such as phase-shift masks have helped to extend exposure resolution below the wavelength of the exposure radiation. Nevertheless, resolution of extremely small features requires correspondingly short wavelengths of radiation. Accordingly, use of X-rays for lithographic exposure by exposing a mask in close proximity to the wafer is known but not widely used due to the requirement for fabrication of an exposure mask at the same minimum feature size as the final desired pattern. Optical and electron beam projection systems can, however, achieve such image pattern size reduction in the exposure pattern relative to feature sizes in a reticle which establishes the pattern to be exposed. Between these two techniques, the manufacturing process for reticles for electron beam projection is much less developed than for optical reticles and, perhaps more importantly, require many more exposures to form a complete integrated circuit pattern since the exposure field at the chip is comparatively more limited in electron beam projection systems. Therefore, there is substantial continued interest in optical lithographic exposure systems and extending their capabilities to shorter wavelengths, such as extreme ultraviolet (EUV).
EUV wavelengths are generally considered to be in the range of about 12 to 14 nanometers and more specifically within a range of less than one nanometer in a band centered on 13 nanometers. At such wavelengths, most imaging materials which are transparent in the visible spectrum and which are suitable for lenses are substantially opaque to the imaging radiation. Therefore, optical systems have been developed and are known which have only reflective elements. To achieve the necessary resolution and uniformity of performance over the illuminated field, the optical aberrations associated with the properties of the reflective elements must be strictly controlled. The maintenance of high manufacturing yield in the above-discussed exemplary environment thus requires not only high stability of the optical system but frequent measurement and adjustment to assure an adequately high level of optical performance of the system.
While techniques of measurement of wave-front aberrations are well-known and sufficient to accurately characterize the performance of optical systems and elements thereof, practical arrangements for conducting such measurements are difficult and complex. For instance, measurements cannot be made on axis or within the exposure/projection field during an exposure without interference with that exposure (e.g. by casting shadows or otherwise occupying a portion of the focus plane of the system where the substrate is located). Measurements performed between exposures may not be regarded as measurements of optical performance during the exposure and do not directly characterize the lithographic image, itself, but are often the only practical solution at the current state of the art even though sources of error may be introduced. Optical performance generally degrades with increasing distance from the optical axis of the system and, as a practical matter, it is desirable to use as much of the field where sufficient precision, resolution and freedom from aberrations can be maintained for projection of the desired image.
Active optics, sometimes referred to as adaptive optics, are known but have not been widely used to date in lithographic applications. Active optics involve the ability to change the overall or local shape of optical elements to alter the optical properties of the element. The article “Active Optics: A New Technology for the Control of Light” by John W. Hardy, Proc. of the IEEE, Vol 66, No. 6, June, 1978, provides an overview of this technology and is hereby fully incorporated by reference. In particular, some general suggestions for provision of mechanical arrangements for achieving localized or generalized deformations of reflecting optical elements to achieve different optical effects such as compensating for atmospheric turbulence are provided. However, mechanical actuators which deflect portions of the optical element such as are disclosed therein may present numerous problems of stability, hysteresis and the like and may be unsuitable for optical element deformations which may be only a relatively small fraction of a very short wavelength. In addition, if measurements of the optical system properties can only be performed at intervals between exposures, the stability of the optical element adjustment becomes critical.